The present invention relates to methods and devices for determination of the angular offset about a longitudinal axis between two primarily similar optical bodies or fibers which each one is axially asymmetric as to its optical properties, and in particular contains at least one eccentrically placed and optically interfering region extending in the longitudinal direction of the fibers, and for aligning these bodies or ends so that the positions of the axial asymmetries coincide, that is in particular the positions of the optically interfering regions coincide angularly about the longitudinal axis of the fibers and the optically interfering regions can be positioned aligned with each other and against each other and then splicing the fiber ends to each other maintaining the alignment of asymmetries, that is in particular of the eccentric, optically interfering regions. In particular the invention relates to methods and devices for determination of the angular offset between the polarization axes of two ends of PM-fibers and for aligning these ends so that the polarization axes coincide and then splicing the fiber ends to each other with a maintained coincidence of the polarization axes so that hereby the polarization of light is well conserved at the transfer between the fiber ends. The invention also relates to methods and devices for determination of the angular offset of planes extending centrally through the two cores in two ends of optical fibers of the kind having double or twin cores and aligning these ends so that the planes coincide or in any case are parallel each other and then splicing the fiber ends to each other maintaining the alignment or parallelism of the planes through the cores.
Optical fibers of standard type comprise a cladding having an essentially circular-cylindrical outer envelope surface and a thin fiber core which is placed rather centrally in the cladding and in the ideal case is located along the longitudinal axis of the outer cylindrical surface and thus has the same longitudinal axis as it. Different methods have been developed to splice fibers having more or less eccentrically located cores, see for instance our earlier Swedish patent applications No. 9100978-7, "Splicing optical fibers", filed Apr. 3, 1991, and No. 9201235-0 "Control of arc fusion in splicing optical fibers", filed Apr. 16, 1992. In these methods no rotation of the fiber ends is required in the splicing operation but only an offset in two lateral directions of one fiber end in order to obtain an alignment of the cores, these methods being in particular intended for the case with somewhat eccentrically located fiber cores. Then a small offset of the exterior surfaces of the claddings is obtained in the finished fiber splice, which is seen as a small step in the longitudinal direction of the spliced fiber. If a rotation thereby of one of the fiber ends also could be introduced, also an alignment of the exterior surfaces of the claddings could be obtained.
Optical PM-fibers are used in such contexts where the polarization state in the transfer of the information through the optical fiber must be strictly controlled, e.g. in sensor contexts.
Commercially available polarization maintaining fibers are constructed as conventional optical fibers having a centrally located core and a surrounding cladding with a cylindrical exterior surface. There is in addition in the cladding, as seen in a cross section, two essentially identical regions of highly doped glass, usually silicon glass doped with B.sub.2 O.sub.3, so-called stress zones, stress concentration zones or stress generating zones, which are located at opposite, diametrically opposite sides of the fiber core. Two such diametrically opposite regions located symmetrically about the longitudinal axis of the fiber extend along the whole fiber. In elliptical jacket fibers there is an ellipsoidal zone centrally inside the cladding, concentric with the core.
In a polarization maintaining fiber there are two polarization modes perpendicular to each other for light transferred in the fiber. They have their magnetic and electric field vectors located along either one of the two perpendicular polarization axes of the fiber, which are also perpendicular to each other, one of which, as seen in a cross section through the fiber, extends centrally through the stress concentration zones.
In the connection of such PM-fibers to each other it is naturally important that the stress zones of the two ends of the fibers are located opposite to each other before the actual splicing procedure so that the polarization axes of the fibers are aligned with each other. A good alignment results in a low cross talk of the polarization modes for light passing through the splice, that is a higher extinction ratio can be achieved.
Optical twin-core fibers having two cores where the cores are designed in the same way as for single mode fibers but are located for instance essentially symmetrically along a diameter plane in the surrounding circular cylindrical cladding constitutes a material in the research of many linear and non-linear phenomena which are based on interaction between the evanescent fields of the basic modes of the cores. They comprise simple beam splitters, fiber sensors and non-linear switches.
A large disadvantage associated with the use of such fibers is, however, the difficulty both in exciting and detection of signals in the two cores owing to their small size and owing to the fact that they are located relatively close to each other. A typical core radius in a fiber having two cores is about 3-4 .mu.m and a typical distance between the two cores is of the magnitude of order a few radii of the core. It is impossible to accomplish a but joint between an optical fiber having a single core of standard type to a fiber having double cores and between two fibers having double cores by means of the conventional splicing methods without performing a rotation of one of the fiber ends which are to be spliced.
A method which has been used to overcome this problem is to use large optical elements and lenses for focusing the input light to the cores. Such methods, however, suffer from high losses in the introduction (7-8 dB) of light, what together with the disadvantage of using large optical components, for instance due to their insufficient stability, make them unsuited for practical use.
Optical PM-fibers and twin-core fibers have the common characteristic of a lacking axial symmetry considered as optical bodies, that is there are non-axial longitudinal optical inhomogeneities or optically interfering regions extending along the fiber. To perform a splice to a similar optical fiber they must be rotated through a measured and/or calculated angle about their longitudinal axes to align the asymmetric regions with each other.
In our earlier Swedish patent application No. 9300522-1, "Alignment and splicing of optical PM-fibers", filed Feb. 17, 1993, it is disclosed how an optical PM-fiber can be given a definite angular position about its longitudinal axis and how this positioning can be used to provide good splices between two optical PM fibers. In the determination the fiber is illuminated with light and the lens effect is observed therein, i.e. the light intensity is determined for light passing through the fiber. A light intensity curve perpendicular to the fiber axis then generally has a maximum corresponding to the core or the central region of the optical fiber. Outside this maximum there is a region having a lower light intensity but where the light intensity still can be rather constant on the said line. Regions outside the exterior surface of the fiber will have a light intensity approximately corresponding to the light intensity without a fiber. The lens effect is constituted by the contrast of the central region having a high light intensity and the region located most adjacent thereto. In order to achieve a positioning a fiber is rotated so that the lens effect will be either maximal or minimal.
In U.S. Pat. No. 5,013,345 for Itoh et al. a method of aligning optical PM-fibers is disclosed utilizing high-precision, costly optical elements, where first one fiber is observed in a predetermined direction to form a reference image, and then the ends of the PM-fibers are both rotated and observed during the rotation, the rotation a fiber end being interrupted when the image thereof coincides with the reference image.